Dampers find application inter alia as a closure member in the flue-gas ducts of large boilers, cement kilns and the like. They are also often used in gas turbine installations for the combined generation of electricity and steam where the damper is placed in the duct carrying the hot exhaust gases from the turbine driving an electric generator to a boiler, so that the boiler can be put out of operation by closing the damper while the gas turbine remains in operation. In this case there is also provided a bypass duct, likewise equipped with a damper, which leads to a chimney.
Dampers of the aforesaid type have relatively large dimensions, for instance, 3 .times. 2 to 6 .times. 4 m, while the pressure level against which the damper is to provide a closure may be 300-500 kp/m.sup.2.
When fully open such dampers must create an acceptably low resistance to flow, and in the closed position ensure substantially gas-tight closure with losses through leakage not exceeding a few tenths of a percent of the total gas flow.
In designing such damper assemblies one problem which is encountered is that in operation relatively large temperature differences, and, consequently, differences in thermal expansion often occur between the damper frame and the blade actuating linkage outside the duct. For example, when the damper blades and their shafts are at a temperature of 500.degree. centigrade the temperature of the damper frame may be 300.degree. centigrade and that of the actuating linkage for the damper blades 50.degree. centigrade. In general this linkage comprises a coupling rod, which consists of adjustable parts interconnecting the successive blade arms and connected at one end to the actuating device that may be operated pneumatically, electrically or manually. If the individual parts of the coupling rod are set so that when cold the blade arms of the damper extend parallel to each other, when the damper frame is hot, this parallel setting is lost owing to the differential thermal expansion of the hot damper frame and the relatively cool coupling rod. It follows that if the damper blades are set for a perfectly tight closure under cold conditions, there will in the hot state, be an angular displacement of the damper blades which increases progressively from the damper blade on the frame side nearest to the actuating mechanism to the blade on the opposite frame side. This may give rise to substantial open gaps between the individual damper blades, so that the desired gas-tight closure by the damper can no longer be assured at all temperatures.
Various proposals have already been made for solving this problem. Thus, for example, it has been suggested that the coupling rod should be so designed and arranged that the individual damper blades would be pulled closer together as the damper frame becomes hotter, the blades being given under cold conditions of the frame, such open bias as to sealingly bear against each other precisely at the expected operational temperature. This proposal, however, is problematic inasmuch as the actual temperature difference between the damper frame and the coupling rod is difficult to predict, so that the desired gas-tight sealing in the closed position of the damper is not reliably attainable. Moreover, the temperature of the gases flowing through the duct may vary according to the operational conditions Another proposed solution envisages arranging resilient elements in the parts of the coupling rod extending between the blade arms. This has the disadvantage, however, that the rigid coupling between the damper blades and actuating device is lost making it uncertain whether the blades will fully close if, for instance, the blade shafts move stiffly in their bearings. Nor does this construction provide any guarantee that when the actuating device is set at a "fully open" position, the damper blades will actually assume the positions in which their resistance to flow is reduced to the lowest possible value.